Hi. A quadrilateral and square both have 360 degrees. For a square, which has 4 right angles, all angles are 90 degrees. Even if you didn’t know a right angle was 90 degrees, you would take the total, 360 degrees, and divide it by the 4 equal angles, giving you 90 degrees. So your answer is “A”.
A quadrilateral can have many combinations of 4 angles that total 360 degrees. To find a missing angle in this case, you need more information about the other angles.